Plug-in hybrid electric vehicles (PHEVs), similarly to conventional hybrid vehicles, include an electric propulsion system working alongside an internal combustion engine to provide motive power for the vehicle. As a result of the common features, PHEVs share many advantages with conventional hybrid vehicles in terms of driving efficiency.
As with conventional hybrids, in a plug-in hybrid electric vehicle the architecture can either take a parallel arrangement or a series arrangement. In a parallel arrangement, both the engine and the electric propulsion system can supply torque directly to a vehicle transmission. Typically the vehicle alternates between the two sources of torque, although at certain times the two sources are used in combination, for example during acceleration from low speed. In pure series arrangements, torque is supplied to the transmission by the electric propulsion system at all times, and the engine is used only as a generator to provide electrical power to the electric propulsion system. In other arrangements, such as ‘powersplit’ or ‘series-parallel’ configurations, the electric propulsion system and the engine can drive the vehicle wheels independently or in combination as required, with the engine acting as a generator when needed.
Relative to conventional hybrid vehicles, PHEVs offer the advantage that a battery of the vehicle can be charged from an external power source when the vehicle is not in use, in the same manner as an electric vehicle. As with electric vehicles, this allows a PHEV to operate in a purely electric mode over a significant distance, known as its ‘all-electric range’. In contrast, in a conventional hybrid vehicle the total electrical energy available for motive effort is much lower, since all electrical energy stored in the vehicle battery is recovered internally, for example through regenerative braking. Therefore, the distance that a conventional hybrid vehicle can cover in a purely electric mode is much more restricted than for a PHEV.
PHEVs are typically operated in two distinct modes: a charge depletion mode, in which battery charge is used at a relatively high rate; and a charge sustain mode, in which the vehicle is operated so as to maintain the battery charge within a defined tolerance band. These modes of operation are each described in more detail below.
The charge depletion mode corresponds to pure electric operation in which the internal combustion engine is inactive, and the vehicle is entirely driven by the electric propulsion system. Therefore, in this mode the level of charge stored in the vehicle battery (referred to hereafter as the ‘state of charge’, or ‘SoC’) is consumed relatively quickly. The rate at which charge is depleted varies according to the load that is applied to the electric propulsion system, which is primarily determined by the way in which the vehicle is driven. For example, aggressive driving characterised by hard acceleration depletes battery charge more rapidly, with more charge being expended per mile of travel, than more sedate driving. Therefore, aggressive driving reduces the all-electric range of the vehicle. Similarly, the load applied to the PHEV will also influence the all-electric range, for example if towing a load, or if driving on an inclined surface. A further consideration is that internal electrical loads, particularly air conditioning systems and entertainment systems, can have a significant effect on the all electrical range.
On starting the engine, the vehicle enters charge depletion mode provided that the initial SoC of the battery is sufficient. While the vehicle operates in charge depletion mode the SoC drops gradually until it reaches a minimum level below which the battery cannot support continued electric operation. At this point, the vehicle switches to the charge sustain mode. As the rate at which charge is depleted is dependent on the manner in which the vehicle is driven, the point at which the transition between modes occurs varies, and is not known in advance. For this reason, the charge sustain mode does not, for example, activate after a certain time period or a certain distance, but is instead triggered with reference to the ability of the battery to support driver demand, for example represented by a predetermined threshold level of charge in the vehicle battery.
Upon entering charge sustain mode the internal combustion engine is started, and the PHEV operates in generally the same manner as a conventional hybrid: for a parallel arrangement the engine becomes the primary source of motive power for the vehicle, and the electric propulsion system is used in parallel with the engine for optimal overall powertrain efficiency. For a series arrangement, the electric propulsion system continues to drive the transmission, but the engine charges the battery to compensate for subsequent electrical power demands.
In this mode the vehicle is controlled so as to maintain the SoC of the battery close to the threshold value that is used to trigger the charge sustain mode initially. This means that when the electric propulsion system is used, for example to aid in moving the vehicle away from a stationary position, the battery charge is replenished from internal sources such as regenerative braking or direct generation from the internal combustion engine.
It is noted that the vehicle continues in charge sustain mode until the battery is next charged from an external power source to raise its SoC above the minimum level. This is primarily because using electrical energy supplied from a grid is typically more economical and more energy efficient than using the engine to charge the electric machine; it would be relatively inefficient and costly to fully recharge the battery using the engine as a generator. Therefore, it is typically preferable to use the engine to maintain the SoC at a minimum level, within a certain tolerance band, and then recharge fully from an external source when the vehicle is not in use.
A problem arises in the PHEV arrangement in that when the engine is started during the transition between the charge depletion and charge sustain modes, components of an engine exhaust gas after-treatment system attached to the engine, such as catalytic converters, are at approximately ambient temperature. The skilled reader will appreciate that catalytic converters do not act to catalytically convert pollutants in exhaust gases until operating in excess of 550° C. It takes approximately 20 seconds for this temperature to be reached from an ambient of 20° C. if the engine is operating specifically for the purpose of heating exhaust gases.
Of particular concern are NOx emissions from the engine, and which the engine exhaust after-treatment system is arranged to manage. Many countries apply restrictions to vehicle NOx emissions, and if the vehicle operates with a catalytic converter below the optimal temperature for a prolonged period it is possible that the emissions may exceed defined limits in those countries.
In a conventional combustion engine powered vehicle, there is typically a period of engine idling following engine start before the vehicle moves away. The catalytic converter warms during this time such that it is effectively treats the exhaust gasses when load is subsequently applied to the engine. Warming of the exhaust gas after-treatment system may be optimised, for example, by using a high engine idling speed while a crankshaft of the engine is not coupled to a driveline of the vehicle, and/or by tuning the combustion to optimise generation of heat.
In contrast, in the PHEV arrangement, load could be applied to the engine almost immediately after engine start, and so the catalytic converter has not had time to warm. There is therefore a risk of exceeding emissions limits for a short period following transition from charge depletion mode to charge sustain mode.
It is against this background that the present invention has been devised.